Borehole instability in shales is regarded as the prime technical problem area in oil and gas well drilling. It is one of the largest single sources of trouble-time associated with drilling, with cost estimates for the industry on the conservative order of magnitude of $500M/year. Shales are ill-defined, heterogenous media ranging from weak clay-rich gumbos to highly cemented shaly siltstones, that have as a common denominator a low-permeability matrix that consists at least partially of clay minerals. Both mechanical and physico-chemical forces act on a downhole shale system. The former include the in-situ stresses, the pore-pressure and mechanical forces in the cementation that may develop in response to tensile or compressive loading. The physico-chemical forces in the clay parts of the shale system include the van der Waals forces and double-layer repulsion and, at small clay platelet distances, a variety of short range forces.
Shales fail if the effective state of stress overcomes the strength of the material. Drilling fluids may induce failure by changing the stress state and/or the material's strength. An obvious source of shale formation instability is the application of inappropriate mud weights. More complex are shale instability problems that derive from shale/drilling fluid communication and interactions. The main driving forces are hydraulic pressure and chemical potential gradients that transport water molecules and/or solutes between the drilling fluid and the shale, and thereby impact shale/pore fluid interaction, pore-pressure, and near-wellbore water content and cementation integrity. It is the nonideal nature of shale (that ill-defined heterogenous media that represents 50-60% by volume of the formation drilled) that prevents the optimization of drilling fluids such that the pore pressure within the rock matrix and the strength of rock matrix itself is controlled to prevent borehole instability problems.
The present invention seeks to provide drilling fluid additives which overcome the above described problems.